The semiconductor diode laser is a p-n junction device which lases when a forward bias voltage of at least 1.5 volts is applied to the device. The voltage drives either holes or electrons or both across the p-n junction and when the holes and electrons recombine they emit light. For an instant before the holes and electrons recombine, they can be "stimulated" by light to emit more light coherently. This stimulated emission phenomenon is equivalent to providing amplification and is related to the first of two requirements for laser oscillation. Specifically, a first requirement is that there be sufficient gain or amplification of the light within the laser to overcome all losses. The second requirement for laser oscillation is an optical feedback mechanism. Optical feedback is provided in conventional diode lasers by simply "cleaving" the faces of the semiconductor crystal. These cleaves form plane parallel mirror-like surfaces which reflect a portion of the light back into the region of the p-n junction. The reflected light is amplified and the energy density within the laser continues to build-up to produce the very intense laser beam.
Several problems have thus far tended to reduce the usefulness and versatility of the described "cleaved-faced" diode laser. First, these diode lasers often fail within tens to hundreds of hours of usage because of damage caused by the high intensity of the light incident on the cleaved mirrors. Secondly, and equally important, no means is known for integrating these diode lasers into an integrated optical system.
To overcome those problems, lasers which do not require cleaved crystal faces to provide feedback have been proposed. The first, which in its various implementations has been referred to as distributed feedback or distributed Bragg reflectors, utilizes a grating-like structure within or adjacent to the active (lasing) region. The grating consists of hundreds of equally spaced corrugations each of which acts as a tiny reflector or mirror. The grating spacing is an integral multiple of the light wavelength generated within the laser such that the reflections from the corrugations are in phase whereby the energy density within the active region builds up to produce the very intense laser beam. A second type of laser not requiring cleaved crystal faces uses an index discontinutiy, which can be created by etching, etching and regrowth, or simply growth.
A drawback of all of the lasers described is that the reflector or feedback mechanism is not readily available to electrical control. A laser with such electrical control would lend itself greatly to integration into complex but easily manufactured multi-component integrated optical circuits.